Generally, ceramic electronic devices have a ceramic substrate and electrodes disposed on the substrate, and such electrodes are formed of a metallic material. Such ceramic electronic devices include, for example, multi-layer ceramic capacitors equipped with an Ni electrode, a Pd electrode, or a Pt electrode; low temperature co-fired ceramic (LTCC) parts equipped with an Ag electrode, a Cu electrode, or an Ag—Pd electrode; piezo-actuators having a Pd electrode; semiconductor packages employing a W electrode; and spark plugs having an Ir electrode or a Pt electrode.
Among the above metal elements, Ni, Cu, and W must be fired with a ceramic substrate in a controlled atmosphere. Thus, difficulty is encountered in attaining characteristics intrinsic to the target ceramic substrate, and production cost rises, which is problematic. In the case of Ag, which has a low melting point (962° C.), the material of the ceramic substrate is limited. In addition, when the substrate is fired at low temperature, characteristics of the ceramic substrate may be impaired. Noble metals such as Pd, Ir, and Pt are expensive materials, making application of such material to wide electrodes difficult.
Patent Document 1 discloses, as an oxide electrode material, a lanthanum cobalt oxide having such a negative temperature characteristic that the resistance thereof is high at ambient temperature and lowers with rising temperature. Patent Document 2 discloses a lanthanum cobalt oxide having such a negative temperature characteristic that the resistance thereof is high at room temperature and the absolute value of B constant is large at high temperature. However, the conductive oxides disclosed in Patent Documents 1 and 2 exhibit high resistivity at room temperature; i.e., poor conductivity.
In the case where an electrode of a ceramic electronic device is formed from metal, the aforementioned problems occur. Thus, the present inventors previously tried to employ an oxide (ceramic) electrode instead of a metal electrode. However, conventional oxides have a conductivity which is considerably lower than that of metallic material, and a large absolute value of B constant (temperature coefficient). Therefore, difficulty is encountered in replacing a metal electrode with a ceramic electrode. Meanwhile, ruthenium oxides (e.g., RuO2 and SrRuO3) are known to have high conductivity, but are problematically expensive. Under such circumstances, the present applicant discloses in Patent Document 3 an oxide sintered body which has high conductivity and a small absolute value of B constant (temperature coefficient) and which is suited for a conductive material. Also, Patent Documents 4 to 6 disclose various perovskite-type conductive oxides.